Sera from 5 cotton workers were preincubated with extracts of strains of B. pumilus (M11 and M38) and added to Vero cell tissue-cultures. The presence of the sera made little difference to those cells treated with the M38 extract but the M11 extract mixed with serum from the worker with Grade 3 byssinosis showed an enhanced effect suggesting a lack of antibody or a defective response.